^^ This is absolutely true. Almost all the teachers I had during O Levels tutored on the side to supplement their income. It wasn't only the case that some taught better during their afternoon/evening coaching sessions, they also provided many invaluable past papers. It's detrimental to the quality of the school system and ratchets up the price of education considerably.
In any case, I'm of the opinion that a student should only be tutored if they genuinely struggle with a subject. I used to struggle with Maths and my tutor, a DU grad student, helped me overcome them. And even if tutoring is required, only those who don't have a stake-hold in our education system should be allowed to tutor. By that I mean, A level students, Uni students, maybe even private coaching centers.